Deoxidation of Valve Metal Powders

ABSTRACT

Deoxidation of valve metal powders, in particular of niobium powders, tantalum powders or their alloys, by treating the valve metal powder with calcium, barium, lanthanum, yttrium or cerium as deoxidising agent, and valve metal powders that are distinguished by a ratio of the sum of the contents of sodium, potassium and magnesium to the capacitance of less than 3 ppm/10,000 μFV/g.

The invention relates to a process for the deoxidation of valve metalpowders, in particular of niobium powders, tantalum powders or theiralloys, by treating the valve metal powder with a deoxidising agent fromthe group calcium, barium, lanthanum, yttrium and cerium, and to valvemetal powders distinguished by a low content of sodium, potassium andmagnesium.

Valve metals, which are to be understood as being especially niobium andits alloys, tantalum and its alloys, as well as the further metals ofgroups IVb (Ti, Zr, Hf, Vb (V, Nb, Ta) and VIb (Cr, Mo, W) of theperiodic system of the elements, and their alloys, are widely used inthe manufacture of components.

Particular mention is to be made of the use of niobium or tantalum inthe manufacture of capacitors, especially of solid electrolytecapacitors. In the manufacture of niobium or tantalum capacitors thereare conventionally used as starting material corresponding metalpowders, which are first compressed and then sintered in order to obtaina porous body. This body is anodised in a suitable electrolyte, wherebya dielectric oxide film forms on the sintered body. The physical andchemical properties of the metal powders used have a critical influenceon the properties of the capacitor. Critical characteristics are, forexample, the specific surface, the content of impurities and, as themost important electrical parameters the specific capacitance at a givenforming voltage U_(f). The specific capacitance is generally given inthe unit microfarad*volt per gram (μFV/g).

General trends in circuit design in the electronics industry are towardsever higher clock frequencies at ever lower operating voltages withminimal electric losses. For the solid electrolyte capacitors used insuch applications this means that ever lower forming voltages are usedand at the same time ever lower leakage currents are required.

Valve metal powders which are to be used in the manufacture ofcapacitors must therefore meet ever higher demands, with the content ofimpurities being of great importance. This applies, for example, to thecontent of oxygen in the valve metal powder, which must not be too high,but also to metallic impurities, which have a decisive influence on theleakage current properties of the capacitor. Such impurities areespecially Na, K, Mg, but also C, Fe, Cr, Ni.

However, the impurities Na, K and Mg in particular are introduced duringpreparation of the valve metal powders owing to the process that isused. Thus, for example, the preparation of tantalum powder is generallystill carried out today according to the reduction, known from U.S. Pat.No. 2,950,185, of K₂TaF₇ with sodium or potassium, which results in highcontents of sodium and potassium in the product.

According to U.S. Pat. No. 4,141,720, tantalum powders having a highoxygen and sodium content can be worked up by adding K₂TaF₇ and alkalihalides and heating the reaction mixture. The contents of oxygen, sodiumand potassium can be reduced in that manner. However, even the powdersso treated have a sodium content of from 10 to 87 ppm and a potassiumcontent of from 112 to 289 ppm.

For the preparation of tantalum powder having a high specific surfaceand a minimal content of sodium and potassium, U.S. Pat. No. 5,442,978proposes reducing highly diluted K₂TaF₇ by the stepwise addition ofsodium, the addition being carried out at a high rate. According toExample 1 it is possible in this manner to obtain a tantalum powderhaving a sodium content ≦3 ppm and a potassium content <10 ppm. However,a deoxidation step is necessary to adjust the oxygen content. To thatend, the tantalum powder is mixed with magnesium and then heated, as aresult of which magnesium is introduced into the tantalum powder.

In addition to the reduction of fluoride salts of the valve metals withalkali metals, oxides of the valve metals are increasingly being used asstarting material recently, which oxides, as described in U.S. Pat. No.6,558,447 B1, are reduced with gaseous magnesium to form thecorresponding valve metal. The content of alkali metal can be kept lowin this manner. However, there is an increased introduction ofmagnesium. In addition, this procedure generally requires a deoxidationstep to reduce the oxygen content after the reduction, whereby themagnesium content in the valve metal powder increases further.

Owing to their high ionic conductivity and the formation of crystallinephases with the dielectric layer of amorphous valve metal oxide producedduring capacitor manufacture, the impurities sodium, potassium andmagnesium cause an increased leakage current in the electric field or onthermal loading during the processing process of the capacitormanufacturer. This is particularly pronounced in the case of the everthinner valve metal oxide layers of <100 nm which capacitors have today.(1 V forming voltage corresponds, for example, to about 2 nm tantalumoxide film thickness).

The object of the present invention is accordingly to provide aneconomical process for the preparation of valve metal powders whichmakes available valve metal powders that are distinguished by a lowcontent of the elements sodium, potassium and magnesium, which arecritical for the residual current of a capacitor. During capacitormanufacture, such valve metal powders form very uniform amorphous oxidelayers at a high specific charge (>35,000 CV/g).

The object is achieved by subjecting the valve metal powder to adeoxidation step in which a deoxidising agent having low ionic mobilityis used.

The invention accordingly provides a process for the deoxidation ofvalve metal powders, wherein calcium, barium, lanthanum, yttrium orcerium is used as the deoxidising agent.

The process according to the invention permits the preparation of valvemetal powders that have a very low content of impurities having highionic conductivity.

As a result, no crystalline phases form with the resulting valve metaloxide during further processing of such valve metal powders tocapacitors, so that defects in the oxide lattice and high residualcurrents are avoided.

The process according to the invention is suitable for the deoxidationof a wide variety of valve metal powders. Preference is given, however,to the deoxidation of niobium powder, tantalum powder orniobium-tantalum alloy powder, particularly preferably tantalum powder.

Accordingly, the valve metal is preferably tantalum.

According to the invention, calcium, barium, lanthanum, yttrium orcerium is used as the deoxidising agent. Calcium or lanthanum ispreferably employed, particularly preferably calcium. The valve metalpowder to be deoxidised is mixed with the deoxidising agent.

This mixture of the valve metal powder with the deoxidising agent isheated to a temperature above the melting point of the deoxidisingagent. It is preferably heated to a temperature that is at least 20° C.above the melting point of the deoxidising agent used.

If calcium is used as the deoxidising agent, the deoxidation ispreferably carried out at a temperature of from 880 to 1050° C.,particularly preferably at a temperature of from 920 to 1000° C. Whenlanthanum is used, the preferred deoxidation temperature is from 940 to1150° C., particularly preferably from 980 to 1100° C.

The deoxidation is preferably carried out at normal pressure. However,it is also possible to work at a lower pressure. The presence ofhydrogen is not necessary in the process according to the invention. Theprocess can be carried out, for example, in vacuo or under inert gas,such as neon, argon or xenon. Nor does the process require a solvent oragent for suspending the solids in a liquid phase, such as, for example,a salt melt, as is conventionally used in the reduction of valve metalcompounds to valve metals.

The amount of deoxidising agent added and the treatment time may varywithin wide limits and depend especially on the oxygen content of thevalve metal powder to be deoxidised and on the deoxidation temperature.

A deoxidation time of from 2 to 6 hours is generally sufficient.Preferably, deoxidation is carried out for from 2 to 4 hours.

There is preferably used a 1.1- to 3-fold stoichiometric excess ofdeoxidising agent, based on the amount that is theoretically required toreduce the oxygen content to 0. It has been shown that it is generallysufficient to use the deoxidising agent Ca in an amount of from 3 to 6wt. % and the deoxidising agent La in an amount of from 6 to 14 wt. %,based on the amount of valve metal powder to be deoxidised, in order toachieve the desired lowering of the oxygen content and of the elementssodium, potassium and magnesium. There are preferably used from 3.5 to5.9 wt. % of deoxidising agent Ca or from 9 to 11.5 wt. % La, based onthe amount of valve metal powder to be deoxidised, particularlypreferably from 4 to 4.7 wt. % Ca or from 10 to 115 wt. % La.

After the deoxidation, the oxides of the deoxidising agent used thatform during the deoxidation are preferably extracted with an acid. Theacid used is preferably nitric acid or hydrochloric acid. It is to benoted that the use of sulfuric acid is to be avoided when calcium isused as the deoxidising agent.

The deoxidation according to the invention is preferably carried out intwo steps. In this case, further deoxidising agent is added to the valvemetal powder after the above-described deoxidation and acid extraction,and the mixture is subjected to the described heat treatment again. Theamount of deoxidising agent is chosen to be lower in the seconddeoxidation step than in the first deoxidation step and preferablycorresponds to a stoichiometric excess of from 1.3 to 2.0, based on theamount of oxygen in the valve metal powder. The deoxidising agent isused in the second deoxidation step preferably in an amount of from 1 to3 wt. % when Ca is used as the deoxidising agent and in an amount offrom 1.5 to 7 wt. % when La is used, based on the amount of valve metalpowder to be deoxidised. Preferably, from 1 to 1.3 wt. % Ca or from 3 to6.1 wt. % La are used as the deoxidising agent, based on the amount ofvalve metal powder to be deoxidised.

The process according to the invention is suitable for the deoxidationof valve metal powders prepared by any method. For example, it ispossible to deoxidise niobium and tantalum powders that are prepared byreduction of a fluoride salt of the valve metal by means of sodium inthe presence of a diluting salt. Such a procedure is known from U.S.Pat. No. 5,442,978, for example.

In the deoxidation of tantalum powders, particularly advantageousresults are achieved when the tantalum powder used as starting materialis obtained by reaction of K₂TaF₇ with sodium in the presence ofpotassium chloride and potassium fluoride under the following reactionconditions: The salt mixture of K₂TaF₇, potassium chloride and potassiumfluoride is placed in a test retort and heated preferably for 6 hours at400° C. in order to remove residual moisture from the salts. The testretort is then heated to a temperature of from 850° C. to 950° C.,preferably from 850° C. to 920° C., particularly preferably to atemperature of 900° C., whereby the salt mixture liquefies. The liquidmelt is stirred under an argon atmosphere (1050 hPa) for the purpose ofhomogenisation. When the reduction temperature is reached, liquid sodiumis added in portions. The total amount of sodium corresponds to a 3 to 6wt. % excess, based on the amount of potassium heptafluorotantalateused. During the addition it must be ensured that the temperature in thetest retort always remains in the range of the reduction temperature(T+/−20° C.). In order to adjust the surface of the precipitatedtantalum powder, an additive that influences the surface tension of thesalt melt, for example anhydrous sodium sulfate, is added to the mixturebefore the first addition of sodium. When the reduction is complete,stirring is continued for a further 0.5 to 3 hours in the range from800° C. to the reduction temperature. Preferably, stirring is continuedfor about 3 hours while simultaneously cooling from the reductiontemperature to 800° C. The reaction material is cooled to roomtemperature and steam is passed through the test retort in order topassivate excess sodium. The retort is then opened and the reactionmaterial is removed and pre-comminuted by means of jaw breakers (<5 cm,preferably <2 cm). The inert salts are then removed by washing, and theresulting tantalum powder is dried. A step of doping with phosphorus canoptionally be inserted here, in which the tantalum metal powder istreated with a (NH₄)H₂PO₄ solution in order to adjust the P content ofthe finished tantalum metal powder. The powder is then exposed to a hightemperature in vacuo. For example, heating is carried out for 30 minutesat from 1250° C. to 1500° C., preferably from 1280° C. to 1450° C.,particularly preferably from 1280° C. to 1360° C. The tantalum powder soprepared is then subjected to the deoxidation according to theinvention.

If is, of course, also possible to use as starting materials valve metalpowders which are obtained by reduction of the valve metal oxides usinggaseous magnesium, as described in U.S. Pat. No. 6,558,447 B1.

It has been shown that it is particularly advantageous to use calcium,barium, lanthanum, yttrium or cerium as the reducing agent in this caseinstead of magnesium.

In a particularly preferred embodiment of the process according to theinvention, therefore, there is used as the valve metal powder to bedeoxidised a valve metal powder that is obtained by reduction of a valvemetal oxide using gaseous calcium, barium, lanthanum, yttrium or cerium.

The procedure for the preparation of the corresponding valve metalpowder is according to U.S. Pat. No. 6,558,447 B1, but calcium, barium,lanthanum, yttrium or cerium is used as the reducing agent.

For the preparation of a tantalum powder, which is preferably used,tantalum oxide (Ta₂O₅) is, for example, placed on a tantalum gauze in atantalum dish. A 1.1-fold stoichiometric amount, based on the oxygencontent in the tantalum oxide, of calcium, barium, lanthanum, yttrium orcerium is placed beneath the tantalum gauze. The reduction is carriedout at a temperature that is sufficiently high to convert the reducingagent to the gaseous state. In order to increase the vapour pressure ofthe reducing agent at a given reduction temperature, it is possible towork at a reduced overall pressure in the reactor. Accordingly, theprocess is generally carried out at an overall pressure in the reactorof less than or equal to 1000 mbar, preferably at an overall pressure inthe reactor of less than or equal to 500 mbar. The reduction temperatureis then preferably from 950 to 1100° C., particularly preferably from980 to 1050° C. In general, reducing times of up to 8 hours aresufficient. When the reduction is complete, the reaction material isremoved and the resulting oxide of the reducing agent is extracted withnitric acid or hydrochloric acid. Analogously to the above-describedprocedure, a P-doping step may also optionally be inserted here.Finally, the valve metal powder so obtained is subjected to adeoxidation according to the invention.

Valve metal powders that are distinguished by a content of Na, K and Mgof less than 3 ppm, based on a capacitance of 10,000 μFV/g, areaccessible for the first time by means of the deoxidation processaccording to the invention.

The invention accordingly further provides valve metal powders that havea ratio of the sum of the impurities sodium, potassium and magnesium tothe capacitance of the valve metal powder of less than 3 ppm/10,000μFV/g.

The ratio of the sum of the impurities sodium, potassium and magnesiumto the capacitance of the valve metal powder is preferably less than 2ppm/10,000 μFV/g, particularly preferably less than 1 ppm/10,000 μFV/g.

The content of the impurities K, Na, Mg is determined after aciddecomposition of the valve metal sample by means of HNO₃/HF. K and Naare determined by the method of flame atom adsorption spectroscopy(FAAS) in an acetylene/air mixture, and magnesium is determined by theICP-OES method (inductive coupled plasma-optical emission spectroscopy).For the acid decomposition, 2 ml of 65 wt. % HNO₃ and 10 ml of 40 wt. %HF are added to 1.0 g of the valve metal sample to be tested, andstirring is carried out for 10 hours at a temperature of 105° C. undernormal pressure. After cooling, 5 ml of 30 wt. % HCl are added, and thevolume of the sample is made up to 100 ml with H₂O. The solution soobtained is then tested by means of FAAS or ICP-OES. The contents thatare determined are indicated in ppm (parts per million).

The capacitance of the valve metal powder is determined by the followingprocedure: Cylindrical compressed bodies having a diameter of 4.1 mm anda length of 4.26 mm and having a compressed density of 4.8 g/cm³ areeach prepared from 0.296 g of a deoxidised valve metal powder, atantalum wire of 0.2 mm diameter being inserted axially into thecompression mould as contact wire before the valve metal powders areintroduced. The compressed bodies are sintered at a sinteringtemperature of from 1330° C. to 1430° C. for 10 minutes under a highvacuum (<10⁻⁵ mbar) to form anodes. The anode bodies are immersed in 0.1wt. % phosphoric acid and formed at a current intensity limited to 150mA to a forming voltage of 30 V. After the current intensity hasdiminished, the voltage is maintained for a further 100 minutes. Inorder to measure the capacitor properties, a cathode of 18 wt. %sulfuric acid is used. Measurement is carried out at a frequency of 120Hz. The residual current is then measured in phosphoric acid ofconductivity 4300 μS. The resulting values of the capacitance of theindividual anode and the residual current of the individual anode arestandardised to μFV/g, where μF=capacitance, V=forming voltage, g=anodemass, or μA/g, where μA=measured residual current and g=anode mass used,or μA/μFV.

The valve metal powders according to the invention preferably have acapacitance of at least 35,000 μFV/g, particularly preferably of atleast 40,000 μFV/g.

The valve metal powders according to the invention are preferablyniobium or tantalum powders, which are optionally doped with one anotherand/or with one or more of the metals Ti, Mo, V, W, Hf and Zr. Furtherdoping elements, such as, for example, phosphorus, are possible.

The valve metal powders according to the invention can be used for awide variety of applications and are suitable in particular for themanufacture of solid electrolyte capacitors.

The examples which follow serve to illustrate the invention in greaterdetail, the examples being intended to facilitate comprehension of theprinciple according to the invention and not to limit it.

EXAMPLES

Unless indicated otherwise, percentages are by weight (wt. %).

Example 1

A tantalum primary powder was prepared at a reduction temperature of900° C. starting from a mixture of 150 kg of K₂TaF₇, 136 kg of KCl, 150kg of KF, 4 kg of a superfine tantalum powder and 300 g of Na₂SO₄ in anickel-coated INCONEL retort by the increment-wise addition of sodium,analogously to U.S. Pat. No. 5,442,978. The tantalum powder was isolatedfrom the cooled and comminuted reaction mixture by washing with weaklyacidified water, a cleaning treatment with a washing solution comprisingsulfuric acid and hydrogen peroxide subsequently also being carried out.The material was doped with 20 ppm of phosphorus using a sodiumdihydrogen phosphate solution containing 1 mg of P per ml of solution.After drying, heat treatment was carried out under a high vacuum at1430° C. Following this, the phosphorus content of the tantalum powderwas adjusted to 60 ppm by means of the sodium dihydrogen phosphatesolution (1 mg of P per ml). The powder exhibited the followingimpurities (in ppm):

Mg: <1 ppm

Na: 0.7 ppm

K: 7 ppm

2 kg of this powder (starting powder) were mixed with 90 g (4.5 wt. %)of calcium powder and heated at 980° C. for 3 hours in a coveredtantalum crucible in a retort tinder an argon atmosphere. After coolingand the controlled introduction of air for passivation, the reactionmaterial was removed and calcium oxide that had formed was removed witha washing solution of dilute nitric acid and hydrogen peroxide solution.The washing solution was decanted oft and the powder on the suctionfilter was washed with demineralised water until free of acid. The driedpowder had an oxygen content of 2831 ppm.

1.8 kg of this powder were then subjected to a second deoxidation step.To that end, 19.2 g of calcium powder (based on the oxygen content, the1.5-fold stoichiometric amount) were mixed into the powder and themixture was likewise heated at 980° C. for 3 hours. After cooling andpassivation, the CaO that had formed was again removed by acid washing,and the powder was washed until free of acid.

The powder so prepared exhibited the following impurities:

Mg: <1 ppm

Na: 1 ppm

K: 8 ppm

The electric test gave a capacitance of 37,419 μFV/g at a sinteringtemperature of 1400° C.

Example 2 Comparison Example

2 kg of the starting powder from Example 1 were mixed with 50 g ofmagnesium turnings (2.5 wt. %) and heated at 980° C. for 3 hours in acovered tantalum crucible in a retort under an argon atmosphere. Aftercooling and the controlled introduction of air for passivation, thereaction material was removed and magnesium oxide that had formed wasremoved with a washing solution of dilute sulfuric acid and hydrogenperoxide solution. The washing solution was decanted off, and the powderon the suction filter was washed with demineralised water until free ofacid. The dried powder had an oxygen content of 2781 ppm.

1.8 kg of this powder were then subjected to a second deoxidation step.To that end, 11.4 g of magnesium turnings (based on the oxygen content,the 1.5-fold stoichiometric amount) were mixed into the powder and themixture was likewise heated at 980° C. for 3 hours. After cooling andpassivation, the MgO that had formed was again removed by acid washing,and the powder was washed until free of acid.

The powder so prepared exhibited the following impurities:

Mg: 8 ppm

Na: 1 ppm

K: 6 ppm

The electric test gave a capacitance of 38,261 μFV/g at a sinteringtemperature of 1400° C.

Example 3 200 g of the starting powder from Example 1 were mixed with 22g of lanthanum powder (11 wt. %) and heated at 980° C. for 3 hours in acovered tantalum crucible in a retort under an argon atmosphere. Aftercooling and the controlled introduction of air for passivation, thereaction material was removed and lanthanum oxide that had formed wasremoved with a washing solution of dilute nitric acid and hydrogenperoxide solution. The washing solution was decanted off, and the powderon the suction filter was washed with demineralised water until free ofacid. The dried powder had an oxygen content of 3045 ppm.

180 g of this powder were then subjected to a second deoxidation step.To that end, 6.5 g of lanthanum powder (based on the oxygen content, the1.5-fold stoichiometric amount) were mixed into the powder and themixture was likewise heated at 980° C. for 3 hours. After cooling andpassivation, the La₂O₃ that had formed was again removed by acidwashing, and the powder was washed until free of acid.

The powder so prepared exhibited the following impurities:

Mg: <1 ppm

Na: 0.7 ppm

K: 8 ppm

The electric test gave a capacitance of 38,093 μFV/g at a sinteringtemperature of 1400° C.

Example 4

A tantalum primary powder was prepared at a reduction temperature of920° C. starting from a mixture of 75 kg of K₂TaF₇, 125 kg of KCl, 225kg of KF, 5 kg of a superfine tantalum powder and 500 g of Na₂SO₄ in anickel-coated INCONEL retort by the increment-wise addition of sodium,analogously to U.S. Pat. No. 5,442,978. The tantalum powder was isolatedfrom the cooled and comminuted reaction mixture by washing with weaklyacidified water, a cleaning treatment with a washing solution comprisingsulfuric acid and hydrogen peroxide subsequently also being carried out.The material was doped with 100 ppm of phosphorus using a sodiumdihydrogen phosphate solution containing 1 mg of P per ml of solution.After drying, heat treatment was carried out under a high vacuum at1280° C. The powder exhibited the following impurities (in ppm):

Mg: <1 ppm

Na: 1 ppm

K: 49 ppm

2 kg of this powder were mixed with 90 g (4.5 wt. %) of calcium powderand heated at 960° C. for 3 hours in a covered tantalum crucible in aretort under an argon atmosphere. After cooling and the controlledintroduction of air for passivation, the reaction material was removedand calcium oxide that had formed was removed with a washing solution ofdilute nitric acid and hydrogen peroxide solution. The washing solutionwas decanted off, and the powder on the suction filter was washed withdemineralised water until free of acid. The dried powder had an oxygencontent of 3700 ppm.

1.8 kg of this powder were then subjected to a second deoxidation step.To that end, 25 g of calcium powder (based on the oxygen content, the1.5-fold stoichiometric amount) were mixed into the powder and themixture was likewise heated at 960° C. for 3 hours. After cooling andpassivation, the CaO that had formed was again removed by acid washing,and the powder was washed until free of acid.

The powder so prepared exhibited the following impurities:

Mg: <1 ppm

Na: 1 ppm

K: 12 ppm

The electric test gave a capacitance of 59,764 μFV/g at a sinteringtemperature of 1400° C.

Example 5

500 g of tantalum pentoxide (Ta₂O₅) having a particle size <400 μm areplaced on a tantalum gauze in a tantalum crucible. The 1.1-foldstoichiometric amount, based on the oxide content in the tantalumpentoxide, of calcium (249.4 g) is placed beneath the tantalum gauze.The tantalum dish is introduced into a scalable retort.

The reduction is carried out for 8 hours under an argon atmosphere at980° C. and at a reaction pressure of 600 mbar. The reaction material isremoved, and the resulting calcium oxide is extracted with nitric acid.The tantalum powder, which has been washed until free of acid, is dopedwith 100 ppm of P on the suction filter using a sodium dihydrogenphosphate solution containing 1 mg of P per ml of solution, and thendried. The tantalum powder so prepared has an oxygen content of 7143ppm.

400 g of this powder are mixed with 18 g (4.5 wt. %) of calcium powderand heated at 960° C. for 3 hours in a covered tantalum crucible in aretort under an argon atmosphere. After cooling and the controlledintroduction of air for passivation, the reaction material is removedand calcium oxide that has formed is removed with a washing solution ofdilute nitric acid and hydrogen peroxide solution. The washing solutionis decanted off, and the powder on the suction filter is washed withdemineralised water until free of acid. The dried powder has an oxygencontent of 4953 ppm.

300 g of this powder are then subjected to a second deoxidation step. Tothat end) 5.6 g of calcium powder (based on the oxygen content, the1.5-fold stoichiometric amount) are mixed into the powder and themixture is likewise heated at 960° C. for 3 hours. After cooling andpassivation, the CaO that has formed is again removed by acid washing,and the powder is washed until free of acid.

The powder so prepared exhibits the following impurities:

Mg: <1 ppm

Na: <1 ppm

K: 2 ppm

The electric test gave a capacitance of 70,391 CV/g at a sinteringtemperature of 1400° C.

1-10. (canceled)
 11. A process for the deoxidation of valve metalpowders which comprises deoxidizing valve metal powders with adeoxidizing agent, wherein the deoxidizing agent is calcium, barium,lanthanum, yttrium or cerium.
 12. The process according to claim 11,wherein the valve metal powder is a niobium powder, a tantalum powder ora niobium-tantalum alloy powder.
 13. The process according to claim 11,wherein the deoxidizing agent is calcium or lanthanum.
 14. The processaccording to claim 11, wherein the deoxidizing agent is calcium and thedeoxidation is carried out at a temperature of from 880 to 1050° C. 15.The process according to claim 11, wherein the deoxidizing agent islanthanum and the deoxidation is carried out at a temperature of from940 to 1150° C.
 16. The process according to claim 12, wherein thedeoxidizing agent is calcium and the deoxidation is carried out at atemperature of from 880 to 1050° C.
 17. The process according to claim12, wherein the deoxidizing agent is lanthanum and the deoxidation iscarried out at a temperature of from 940 to 1150° C.
 18. The processaccording to claim 11, wherein the deoxidation is carried out in twosteps.
 19. The process according to claim 11, wherein the valve metalpowder obtained by reduction of a valve metal oxide with gaseouscalcium, barium, lanthanum, yttrium or cerium is deoxidised.
 20. A valvemetal powder which comprises the ratio of the sum of the impuritiessodium, potassium and magnesium to the capacitance of the valve metalpowder is less than 3 ppm/10,000 μFV/g.
 21. The valve metal powderaccording to claim 20, wherein the ratio of the sum of the impuritiessodium, potassium and magnesium to the capacitance of the valve metalpowder is less than 1 ppm/10,000 μFV/g.
 22. The valve metal powderaccording to claim 20, wherein the valve metal powder is a niobiumpowder.
 23. The valve metal powder according to claim 20, wherein thevalve metal powder is a tantalum powder.
 24. The valve metal powderaccording to claim 21, wherein the valve metal powder is a niobiumpowder.
 25. The valve metal powder according to claim 21, wherein thevalve metal powder is a tantalum powder.